The invention relates generally to sulfur. More particularly, it relates to a novel method of achieving novel superior dispersions of insoluble sulfur in unvulcanized rubber by adding polysulfur to the rubber while the polysulfur is in the rubbery or unhardened state and to the resulting product.
Rubber and similar polymers must be mixed with a selection of other ingredients to develop the properties necessary for specific applications. One of these ingredients is a vulcanizing agent. Elemental sulfur is by far the most widely used vulcanizing agent, especially in tires and other dynamic applications. While certain chemical compounds of sulfur can be used as sulfur donors to accomplish vulcanization, only elemental sulfur is believed to impart the optimum combination of properties for most of the tire. One of the most important properties required in tire rubber is fatigue resistance. The superior fatigue resistance achieved when using elemental sulfur instead of sulfur donors is reported in the xe2x80x9cNatural Rubber Formularyxe2x80x9d pp128-129, and again on pp180-181. Elemental sulfur is used in the rubber industry in two basic forms:
1. Ortho-rhombic (commonly called rhombic) crystals, consisting of molecules containing eight sulfur atoms per molecule in a ring-like structure. This form is referred to as normal, or soluble sulfur. Its main disadvantage is that it xe2x80x9cbloomsxe2x80x9d in the unvulcanized rubber compound.
2. Polymeric sulfur (or polysulfur, as it is sometimes called to distinguish it from organic polymers containing sulfur in their polymer chains) consists of molecules that contain long chains of sulfur atoms, usually thousands of sulfur atoms per molecule. At room and processing temperatures., these chains tend to revert to normal sulfur. This reversion can be deterred by adding certain stabilizing agents in small quantities. The stabilized forms have dominated the market. Polymerized sulfur or polysulfur is referred to as insoluble sulfur. There are no known solvents for insoluble sulfur; hence its name. Its main disadvantage is that it is hard to disperse well in the unvulcanized rubber compound. It is also quite expensive compared to normal sulfur.
The degree of dispersion, in the unvulcanized rubber compound, of all of these compounding ingredients affects the properties of the vulcanized product. This is especially true of the vulcanizing agent. For the vast majority of products, such as tires, the best dispersion gives the-best product because of the homogeneity achieved.
Sulfur exists at room temperatures primarily as rhombic crystals. Other forms of sulfur, such as monoclinic crystalline sulfur, or polysulfur, are the normal primary forms which elemental sulfur assumes at certain higher temperature ranges. At room temperatures, these forms convert, or revert, to rhombic sulfur.
Polysulfur is called insoluble sulfur, especially in the rubber industry, and normal, non-polymeric or rhombic sulfur is called soluble sulfur, because it is soluble to a limited extent in most rubbers. The term xe2x80x9crubberxe2x80x9d as used herein means any sulfur vulcanizable polymer. Sulfur vulcanizable polymers are primarily those polymers having carbon to carbon molecular chain structures, with some double bonds existing in their structure. These polymers are called unsaturated. The double bonds are the sites for sulfur vulcanization. The term xe2x80x9crubberyxe2x80x9d as used herein means masses of matter that are not hard, brittle, or friable, but are plastic and/or elastic. The term xe2x80x9csaturated rubbery polymersxe2x80x9d means those rubbery polymers that do not contain sulfur vulcanizable bonds, such as ethylene-propylene rubber.
The amount of normal or rhombic sulfur that is soluble in rubber increases as the temperature increases. Typical rubber compounds contain from one to three parts of sulfur per one hundred parts of rubber hydrocarbon (rhc). The processing of unvulcanized rubber requires mechanical working of the rubber, which generates heat. The temperatures developed as a result of this processing are usually sufficient to dissolve the typical normal sulfur content. When the rubber cools to room temperature the solubility of the sulfur in rubber is exceeded, and a supersaturated solution ensues. This supersaturated portion of the sulfur tends to migrate to the surface of the rubber and crystallize. This condition is called xe2x80x9cbloomxe2x80x9d and is highly undesirable.
At room temperatures, surface blooming occurs primarily when the concentration of soluble sulfur in the rubber is between the limits of about 0.8 parts and 8.0 parts per 100 parts of rubber hydrocarbon. These limits vary among different compounds. Below the lower limit the sulfur is soluble. Above the upper limit the sulfur drops out of solution in the interior of most rubber compounds, forming micro-crystals throughout the mix. In some rubber compounds these micro-crystals grow to objectionable size, causing nonhomogeneity of properties throughout the vulcanized product.
Polysulfur or insoluble sulfur does not dissolve in rubber, and therefore does not bloom. However, the insoluble sulfur can revert to normal sulfur, and the rate of reversion is a time-temperature phenomenon which increases with temperature. Elemental insoluble sulfur can be stabilized by the addition of various substances, notably the halogens. This stabilized insoluble sulfur tends to remain polymeric at room and processing temperatures but it reverts to normal sulfur at the higher vulcanizing temperatures, thus becoming available for the vulcanization reaction.
Insoluble sulfur is normally supplied by the sulfur manufacturers in discrete particles, or powder. This powder is extremely fine, classically having a reported average particle size of 3 microns. These particles are considerably smaller than the particles usually supplied of normal sulfur. These smaller particles are desired because the dispersion of this form of insoluble sulfur is limited by the particle size supplied, unlike the dispersion of soluble sulfur. This very fine powder presents various processing difficulties. It tends to form dust clouds in the mixing room, which are both a health hazard and a safety hazard. Sulfur dust explosions are a known hazard in the rubber industry. A number of ways to reduce this dusting are mentioned in the literature. Also, the sulfur powder is difficult to disperse in rubber. The individual particles tend to agglomerate. Because of this, the powders are frequently mixed with a portion of a polymer or other matrix materials to form a masterbatch before being added to the final compound. These masterbatches usually contain fifty percent or more sulfur. This processing step adds to the cost. Since these discrete particles retain their identity during mixing, the best possible dispersion is limited by the size of the particles, unless their melting point is exceeded. However, when melted, the rate of reversion is very rapid and the reverted sulfur, of course, blooms, and the advantages of using insoluble sulfur are negated.
The prior art falls in three categories:
1. Insoluble sulfur powders
U.S. Pat. No. 2,419,310 to Belchetz
U.S. Pat. No. 2,419,309 to Belchetz
U.S. Pat. No. 2,579,375 to Grove
These patents deal with insoluble sulfur in a form that has distinct disadvantages. The present invention overcomes these disadvantages.
2. Sulfur donors
U.S. Pat. No. 4,621,118 to Schloman
U.S. Pat. No. 2,989,513 to Hendry
U.S. Pat. No. 2,481,140 to Morris
All of these patents teach a chemical reaction of sulfur with an organic compound to form sulfur donors. The crosslinking achieved using sulfur donors is distinctly different from that achieved using elemental sulfur. No long chain polymers of sulfur are contemplated or achieved. Therefore they are not pertinent.
3. Solutions of normal sulfur
U.S. Pat. No. 1,782,693 to Miller
This patent teaches solutions of normal sulfur in an organic resin. Long chain polymers of sulfur do not go into solution in any known substance. Hence it fails to teach or suggest anything concerning polymeric sulfur.
Recently an xe2x80x9cimproved productxe2x80x9d has been introduced, that has a reported average particle size of 2 microns. The improvement is in the degree of dispersion afforded by the smaller particle size. However, as could be expected, the dusting problem has gotten worse with the xe2x80x9cimproved productxe2x80x9d, and the dispersion is, of course, still limited by the particle size. There is still a need for much better dispersion than is achieved by the xe2x80x9cimproved productxe2x80x9d, or that can be achieved by any product that must rely on the size of a particulate to maximize its dispersion.
It is generally known that rapidly cooling molten sulfur from a sufficiently high temperature produces a mass of polymeric sulfur that is in a rubbery or unhardened state. Sulfur in this rubbery state is in a metastable condition, and upon standing, it becomes hard and brittle. It can then be ground to form a powder. In some processes the molten sulfur is sprayed and thereby cooled, the individual droplets formed in the spraying are also initially rubbery, but are then permitted to become hard and brittle. Special handling is necessary to keep them as individual particles before hardening since the rubbery state tends to make them agglomerate. In other processes, sulfur vapor is forced under pressure into a liquid cooling medium. Again rubbery particles are first formed, and special handling is required until the particles harden. The rubbery unhardened state of insoluble sulfur has been considered undesirable because it has been a free flowing powder which has been desired.
I have discovered that the metastable, unhardened, rubbery or plastic state of insoluble sulfur can be preserved by lowering the temperature of the sulfur below the glass transition temperature of the rubbery mass. Upon returning to room temperature, the mass again becomes rubbery, but upon further standing it becomes hard and brittle. When in the rubbery state this sulfur can be dispersed in rubber to obtain the superior blends or dispersions of the present invention. If desired, the sulfur can be added to the rubber mix while it is still below its glass transition temperature. When exposed to the milling temperature it quickly warms up to the rubbery state.
Other elements in Group VIB of the Periodic Table of the Elements, notably Selenium and Tellurium, have many properties that are similar to sulfur. They have found limited use as vulcanizing agents for rubber, and they polymerize. It is well known that they polymerize themselves and that they form copolymers and terpolymers with sulfur. These polymers exhibit rubbery properties similar to those of the monopolymer of sulfur, and hence can be dispersed in similar fashions as the rubbery polymer of sulfur. All of these polymers are included in the term xe2x80x9cpolysulfur.xe2x80x9d
Other elements, such as Arsenic, can be included in the sulfur polymer chain, and a rubbery state can still be obtained. One skilled in the art could develop many such modifications of the sulfur polymer, and still obtain a rubbery polymer. These modifications are included in the substance of this invention. The terms xe2x80x9cpolymeric sulfurxe2x80x9d and xe2x80x9cpolysulfur,xe2x80x9d as used herein, includes all of the modifications of the polymer that can be obtained in the rubbery state.
There is a need for ways to achieve superior dispersion of polymeric sulfur in rubber in a dust free manner. This need exists whether the sulfur is added to a rubber batch in the lower amounts needed for proper vulcanization, or in the higher amounts used in preparing masterbatches or intermediate batches, which are later added in the proportion needed to achieve the proper quantity of sulfur needed for vulcanization.
It is an object of the present invention to disclose how to achieve superior dispersions of insoluble sulfur in unvulcanized rubber, in a completely dust free manner.
It is a further object of the present invention to disclose novel superior blends made of insoluble sulfur with a diluent which is compatible with sulfur vulcanizable polymers.
It is a further object to disclose a novel dust-free method of preparing such blends.
I have discovered that a superior dispersion of insoluble sulfur is attained if polysulfur is added to the rubber while the polysulfur is in the metastable, unhardened or rubbery state. Previous to the present invention, this state has always been regarded as undesirable. The literature contains many descriptions of ways to harden this rubbery mass, so as to make it usable by prior known art.
I have further discovered that superior preparations of polymeric insoluble sulfur can be prepared by a method which comprises mixing polymeric sulfur while in the rubbery unhardened state with a compatible diluent, which is also compatible with the polymer to be vulcanized. The amount of polysulfur in the blend can vary from about 1% to about 95% by weight of the total blend. However, the preferred blends will contain about 20% to about 95% of the rubbery polysulfur by weight.
Electron Micrographs of a cross section of this superior dispersion, taken by a Leica Cambridge Ltd. S360 Scanning Electron Microscope, Version V0302A, equipped with Dynamic Stereo and Remote Control options, failed to discern any areas of unblended polysulfur. The same device clearly discerned the  less than 1 micron to 3 micron particulates of Crystex Sulfur, the brand of insoluble sulfur that presently dominates the market because it has been the best available, in a 50/50 masterbatch dispersion of Crystex/EPR 707 (a high Mooney Viscosity Ethylene Propylene Rubber). These micrographs also showed the non-uniform dispersion of the sulfur particulates, and showed three-dimensional agglomerates of these particulates up to 29 microns across.
The blends of the present invention, since they are formed with the polysulfur in the rubbery state, eliminate the need for comminution into fine powders, thereby effecting significant economies and eliminating the hazardous formation of dust. It also eliminates the need for the rubbery sulfur polymer to harden, so that it can be processed into a powder, thereby effecting more economies. The blends of the present invention when properly prepared do not harden. They remain in the viscous or rubbery state so as to be readily dispersable by the commercial methods used to mix rubber.
In the preferred practice of the method of the present invention, the conversion process by which soluble sulfur is converted to polysulfur is interrupted when the polysulfur is in the rubbery state, and the rubbery polysulfur is mixed with the unvulcanized rubber, in the proportion needed for proper vulcanization, or with the compatible diluent, at that time.
In another embodiment, a portion of the polymer that is to be vulcanized is mixed or masticated and used as the compatible diluent for the rubbery polysulfur which is blended into it, thereby reducing the number of processing steps even further. A two roll rubber mill or other masticating equipment can be used.
In still another embodiment, normal or rhombic sulfur can be mixed in a relatively high percentage (e.g. 80%) with a compatible diluent, such as EPR (ethylene-propylene rubber, a saturated rubbery polymer) or an unsaturated rubbery polymer, and the sulfur polymerized in situ, by raising the temperature of the mixture to melt the sulfur and then rapidly cooling the mixture below about 113xc2x0 C. the melting point of sulfur. This diluent is of such nature that it will not significantly react with the sulfur under the conditions of the process. This process is best done in a chambered mixer, such as an extended barrel extruder rather than on an open mill. When using the preferred high percentage of sulfur, the molten sulfur tends to separate out of the mixture and runs off the mill. In the preferred extruder, the first zone is heated and subsequent zones cooled.
In another preferred embodiment, the sulfur conversion unit is set up convenient to the intensive internal mixer of the Banbury type or a rubber mill. The rubbery sulfur is added to the final or intermediate batch in the quantities needed for proper vulcanization of the intended final batch. A 1.6 kilogram (3 lb.) capacity laboratory mixer was used.
If desired, the polymerized molten sulfur can be added directly to the rubber batch being mixed in the intensive mixer, using the rubber matrix as the rapid cooling medium to cool the mass and maintain it below the melting point of sulfur or to about 113xc2x0 C. or less. The lower the temperature the more stable and hence better the product will be.
It will be apparent to those skilled in the art that the above objects and other advantages will accompany the practice of the present invention.
In the preferred practice of the method of the present invention, polysulfur in the rubbery state is blended with the compatible diluent, which is rubber, by mixing the rubbery polysulfur and the diluent on a two roll rubber mill and continuing the blending or mastication until a uniform blend is obtained. The amount of the rubbery polysulfur which is used is preferably enough to produce a uniform blend which contains about 40% to about 80% of the polysulfur as based on the total weight of the blend.
The rubbery polysulfur can be prepared by any known manner. It is known that at about 113 degrees C. and above, sulfur melts. The molten sulfur begins to polymerize as the temperature is raised to about 159 degrees C. This is evidenced by a pronounced increase in viscosity. As the temperature is further increased, the viscosity increases sharply to a maximum in a rise of only a few degrees. It then drops off at a fairly steady rate as the temperature is still further increased. It is believed that the mean chain length is at its maximum at the point of maximum viscosity, and that the mean chain length decreases as the temperature rises further. The higher viscosities apparently have not been shown to inversely correlate with the percent of unpolymerized sulfur present. The purer the sulfur, the higher is the maximum viscosity. The curve of the viscosity versus temperature of pure sulfur is reversible if the temperature changes are gradual. This reversibility does not necessarily hold true when other substances, such as certain organic compounds or halogens, are present in small amounts. In pure sulfur, the end of the polymer chain is a free radical. When these other substances are present, either purposefully or accidentally, the free radical is probably xe2x80x9ccappedxe2x80x9d by the other substances. However, for purposes of this application the term xe2x80x9cpolysulfurxe2x80x9d is intended to cover the just-described capped polymers.
If the molten sulfur is cooled gradually from the temperature of maximum viscosity, the drop in viscosity is attributed to the reversion of the polysulfur to predominantly S8. Other sulfur molecules, notably S6, but others from S2 to S23 have been identified.
When the molten sulfur is cooled rapidly from temperatures whereat a significant portion of the sulfur is polymeric, the polymeric form survives to a metastable form, which is rubbery, at room temperature. In pure sulfur, the polymer reverts to predominantly S8 in a relatively short time. This reversion is hindered by the presence of halogens or certain other substances, and seems to be accelerated by the presence of moisture, or alkalinity.
Upon rapid cooling, the molten polysulfur forms a rubbery mass of rubbery polysulfur, which hardens by itself upon standing. This hardening does not necessarily indicate de-polymerization, or reversion. Certain treatments can accelerate this hardening.
The compatible diluent which is blended with the rubbery polysulfur to form the blends of the present invention can be any substance which is compatible with both the rubbery polysulfur and the polymer which is to be vulcanized with the blend. Representative diluents include natural and synthetic rubbers, soaps, petroleum fractions, waxes, wood tar products and plasticizers, such as ethers and esters and their polymers.
The method of preparing the blends can be any of those described herein, as well as other methods which can be used to form sufficiently uniform blends.